About the A-a Gradient
The alveolar-arterial (A-a) oxygen gradient is the difference between the oxygen pressure in the alveoli (PAO2) and the oxygen pressure measured in arterial blood (PaO2). It is used to work out why a patient is hypoxemic: a widened gradient points to a problem inside the lung, while a normal gradient points to a cause outside it. This tool computes the gradient from a single arterial blood gas plus the inspired oxygen and barometric pressure.
The formula
Alveolar oxygen comes from the alveolar gas equation, and the gradient is the alveolar value minus the arterial value:
- Alveolar oxygen: PAO2 = FiO2 x (Patm - PH2O) - PaCO2 / RQ, where PH2O is water vapor pressure (47 mmHg at body temperature) and RQ is the respiratory quotient (assumed 0.8).
- A-a gradient: PAO2 - PaO2, using the measured arterial PaO2 from the blood gas.
- Age-adjusted normal: a common estimate of the upper limit on room air is (age / 4) + 4 mmHg.
Interpreting the result
On room air a normal gradient is roughly 5 to 10 mmHg in healthy young adults and rises with age. A gradient above the age-adjusted expected value suggests impaired gas exchange - ventilation-perfusion mismatch, right-to-left shunt, or a diffusion defect. Hypoxemia with a normal gradient instead suggests hypoventilation or a low inspired oxygen level, such as at altitude.
Assumptions and when to consult a professional
The calculation assumes a water vapor pressure of 47 mmHg, a respiratory quotient of 0.8, and steady-state gas exchange; the age-adjusted normal applies to breathing room air. It is a computation only and not a diagnosis. Any abnormal or concerning result should be interpreted by a licensed healthcare provider in the context of the full clinical picture.